The conventional polymerization of 2-pyrrolidone is performed in the presence of a catalyst system composed of an alkaline catalyst and a small amount of polymerization initiator. Recent prior art teaches repeatedly that an alkali metal in metallic form should not be used as the source of the alkaline catalyst system (U.S. Pat. Nos. 3,721,652, 3,683,046, 3,682,869, 3,681,296, 3,681,295 and 3,681,293) because it gives a catalyst system which is deleterious to the polymerization reaction. On the other hand, the same prior art teaches that alkali metal derivatives, such as the hydroxides, are suitable sources of alkaline polymerization cataylst. For example, the use of a sodium metal-derived catalyst system has been shown to produce a low yield (3.2%) of low-molecular-weight polymer unsuitable for melt spinning, while potassium hydroxide-derived catalyst systems produce, in good yield, a polymer suitable for melt spinning.
The unsuitability of the metallic alkali metals for catalyst preparation has been ascribed to reduction of the sensitive 2-pyrrolidone ring by these very strong metallic reducing agents (K=K.sup.+ +e.sup.-, E.sup.o = 2.925 volts and Na=Na.sup.30 +e.sup.-, E.sup.o = 2.714 volts relative to H.sub.2 = 2H.sup. + 2e.sup.-, E.sup.o = 0.0 volts, see Latimer and Hildebrand, "Reference Book of Inorganic Chemistry", 3d Ed., MacMillian, p. 528). On this basis alone, potassium metal should be an even more deleterious source of the alkaline catalyst system than sodium metal. Furthermore, the alkali metal-derived catalysts are said to be difficult, or even impossible, to purify and are said to lose catalytic activity with time (see U.S. Pat. 3,835,100, Cols. 1 and 9).